1. Field of the Invention
The present invention relates to fixed-type constant velocity universal joints to be used in power transmission systems of automobiles and various kinds of industrial machines, each of which only allows an operational angle displacement between a shaft on the power-transmitting side and a shaft on the driven side.
2. Description of the Related Art
Fixed-type constant velocity universal joints (e.g., Rzeppa constant velocity universal joints well known as Birfield universal joints: BJs) have been used for the connection between drive shafts or the like in automobiles. Typically, the conventional fixed-type constant velocity universal joint (hereinafter, simply referred to as the conventional universal joint) comprises: an outer joint member in which curved track grooves are axially formed on its inner circumferential surface with a spherical shape; an inner joint member in which curved track grooves are axially formed on its outer circumferential surface with a spherical shape; a plurality of balls respectively arranged in ball tracks provided as pairs of track grooves of the inner and outer joint members; and a cage having pockets for respectively holding these balls. More specifically, the plurality of balls are arranged in their respective pockets of the cage at regular intervals in a circumferential direction of the joint.
The center of the track groove of the outer joint member is displaced a predetermined distance from the spherical center of the inner circumferential surface. On the other hand, the center of the track groove of the inner joint member is displaced the same distance from the spherical center of the outer circumferential surface in the direction opposite to that of the outer joint member. Here, such a displacement is referred to as a track offset. Therefore, the ball track formed between the outer joint member and the inner-joint member is shaped like a wedge opened toward one end of the joint in the axial direction. In addition, the spherical center of the inner circumferential surface of the outer joint member and the spherical center of the outer circumferential surface of the inner joint member are located within a common plane, i.e., the central plane of the joint that includes the center of each ball.
In the conventional universal joint, the constant velocity of the joint can be ensured because each of the ball held in the pocket of the cage can be always located in a plane that bisects any operational angle when there is an angular displacement between the outer joint member and the inner joint member. Here, the operational angle refers to an angle formed by a rotational axis of the outer joint member and a rotational axis of the inner joint member.
In recent years, there may be cases where a wheel base is lengthened from the viewpoint of improving the safety of automobile in the event of a crash. In this case, however, there is a need to increase a steering angle for front wheels by providing the universal joint with a higher operation angle. For filling the need for such a higher-angle, there is provided another conventional universal joint, i.e., an undercut-free type universal joint (UJ) in which the track grooves on the opening side of the outer joint member are shaped so as to be in parallel with the axial direction of the joint. In this kind of the universal joint, there is no under cut formed on either of the outer joint member or the inner joint member so that a higher operation angle can be attained.
In each kind of the conventional universal joints (BJ, UJ), it is very important to define how to fill a gap between a pitch circle diameter of the track groove of the outer joint member and a pitch circle diameter of the track groove of the inner joint member (hereinafter, such a gap is referred to as a PCD gap). If the PCD gap is too small, it becomes difficult to insert balls into the respective ball tracks and each of the balls is then difficult to roll smoothly as the binding force upon the ball increases. During the rotation of the universal joint, the rolling movement of the ball is caused with sliding contact between the ball and the ball track. As a result of frictional heating, the temperature in the joint increases and hence the lifetime of the joint decreases. If such a gap is too large, on the other hand, the characteristic features of the joint (such as noise, vibration, harshness (NVH), durability) can be affected by the generation of slapping sounds between the pocket and the ball, or more the increase in vibrations of the joint.
Especially, under high-load conditions, the contact ellipse between the ball and the track groove runs off the track groove. Under such circumstances, it could be chipped from such a portion, resulting in flaking. If the PCD gap is small, it is effective to prevent the contact ellipse from running off the track groove. On the other hand, if such a gap is substantially larger than usual, the ball contact point approaches the PCD gap and hence the contact ellipse becomes easy to run off the track groove.
In each of the conventional universal joints (BJ, UJ), furthermore, it is very important to determine the ratio (F/PCR) between the amount of track offset (F) and the length (PCR) of a straight line segment between the center of the track groove in the outer or inner joint member and the center of the ball. In Japanese Patent Laid-Open Publication No. Hei. 9-317784 (1997), for example, there is disclosed an appropriate range of F/PCR (i.e., 0.069≦F/PCR≦0.121). In this case, the depth of the track groove becomes shallow if the amount of track offset (F) is too large. Thus, the contact ellipse between the ball and the track groove tends to run off the track groove, causing the decrease in allowable load torque at a higher operational angle region. In addition, columns of the cage become narrow, so that the strength of the cage can be also decreased. On the contrary, if the amount of track offset (F) is too small, the track load increases. It causes the decrease in the durability of the joint in addition to lowering of the maximum operational angle.
Typically, the conventional universal joint has six balls (hereinafter, such a joint will be also referred to as a six-ball joint). However, there is an alternative universal joint having eight balls for the purpose of realizing a smaller and lighter version of the joint while retaining at least the same properties (e.g., strength, load capacity, and durability) as those of the joint using six balls. Here, the universal joint having eight balls will be also referred to as an eight-ball joint. The eight-ball joint has its own basic configuration different from that of the six-ball joint. In this case, the set value of the PCD gap may be peculiar to such a configuration. The amount of track offset (F) and the length PCR of a straight line segment between the center of the track groove of the outer or inner joint member and the center of the ball are also adjusted such that the ratio F/PCR can be defined within the above appropriate range. It is noted that the depth of the track groove of the eight-ball joint is smaller than that of the six-ball joint as the diameter of each of the eight balls is smaller than that of the six balls. In the eight-ball joint, the PCD gap exerts a very large influence on the durability of the joint. Consequently, there is a limit to improve the durability of the eight-ball joint by means of adjusting the above F/PCR in the appropriate range.